Conventionally, a bottom end-piece of a nuclear fuel assembly performs three functions.
The first is a function involving mechanical strength since the end-piece forms part of the skeleton of the nuclear fuel assembly and the assembly is supported on the lower core plate thereby.
The second function is a function for distributing the cooling water which flows in the core of the reactor vertically from bottom to top at an ascending rate which is very high.
More precisely, the water enters the core through the lower core plate, then passes through the bottom end-piece, before coming into contact with the outer surfaces of the fuel rods.
The third is a function for retaining debris. When flowing in the coolant circuit of the nuclear reactor, the cooling water accumulates debris which may be present. It is desirable to prevent this debris from becoming wedged between the fuel rods and the elements which retain them, in particular in the region of the lower grid of the skeleton. This debris, which is then subject to significant axial and transverse hydraulic forces in this zone could bring about wear of the claddings of the fuel rods and result in losses in terms of sealing and an increase in the level of activity of the coolant circuit of the reactor.
This function is fulfilled by anti-debris devices which may be integrated in or fitted to the bottom end-piece and may be complex to a greater or lesser extent.
Document U.S. Pat. No. 5,030,412 describes a complex anti-debris device which delimits passages which successively form, in a vertical cross-section, two baffles which allow a higher volume of debris to be retained.
However, bottom end-pieces which are provided with such more complex anti-debris devices lead to a greater pressure drop. This pressure drop becomes evident with a lower level of cooling efficiency for the nuclear fuel rods and the need to compensate for this additional pressure drop by increasing the hydraulic transparency of other components of the nuclear fuel assembly and/or with a greater axial retention force applied to the nuclear fuel assembly by the upper core plate.
Furthermore, the presence within the same reactor core of nuclear fuel assemblies which are provided with bottom end-pieces having a low pressure drop and assemblies which are provided with bottom end-pieces having a higher pressure drop is also problematic. It leads to occurrences of redistribution of the cooling water in the lower portion of the rods of the assemblies, bringing about lateral flows which may reach significant speeds. These lateral flows could lead to damage to the rods owing to friction of the claddings thereof on the grids of the skeleton. A phenomenon of this type is generally referred to as “fretting”.
More generally, such fretting phenomena could occur between the lower grid of the support skeleton and the claddings of the nuclear fuel rods, even when the core is loaded with assemblies having bottom end-pieces which bring about the same pressure drop, owing to the vibrations of the lower ends of the nuclear fuel rods.
These fretting phenomena could damage the claddings, which could cause fission gas or products to be released into the water of the coolant circuit.
In order to overcome this problem of fretting, FR-2 864 324 described a bottom end-piece of the above-mentioned type. Preferably, the lower ends of the nuclear fuel rods are retained axially and laterally in blocks which cover the nozzles for directing the flow of water. This retention and the improved guiding of the cooling water by the nozzles around the nuclear fuel rods, allows the vibrations of the lower ends of the rods to be limited and therefore allows the risk of damage caused by fretting to be reduced.
The bottom end-piece of this document may further be provided with an anti-debris device which is fitted above the network of nozzles for directing the flow of cooling water. In a variant, this device may be integral with the nozzle network.
A bottom end-piece of this type is satisfactory but it appears to be desirable to further increase the capacity thereof for retaining debris.